EXTERNAL ANATOMY OF INSECTS. 407 



iv. With regard to its superficies, the head of insects 

 is generally more or less uneven, though in some cases it 

 presents no inequalities. In many of the Lamellicorn 

 tribes, and a few other individuals, in one sex at least, 

 as has been before observed 3 , it is armed with long 

 horns, or prominent tubercles ; it is often covered with 

 numerous puncta, or pores ; and some of its parts, as the 

 nose, after-nose, &c. in particular groups, marked out 

 by an impressed line b . In many Hymenoptera, Diptera, 

 &c. its upper surface is convex, and the lower concave; 

 in others both surfaces are convex. 



v. It is the most general rule, as to its proportion, that 

 it shall be smaller than either trunk or abdomen ; but in 

 some instances, as in the S. American ant, Atta megace- 

 phala, it is much larger than either. 



vi. By the direction of the head, I mean its inclina- 

 tion with respect to the prothorax. The most natural 

 direction, or that which obtains most generally, is for it 

 to form an angle more or less obtuse with the part just 

 mentioned. This seems to obtain particularly in Coleo- 

 ptera ,■ but in some, as Mylabris, it is infiexed, forming 

 an acute angle with it. In the Heteropterous Hemiptera 

 (Cimex L. &c.) it is generally in the same line with the 

 body, or horizpntal ; and in many Diptera it is vertical. 



vii. We now come to a circumstance which will de- 



a See above, p. 309 — . 



b In the hornet and other wasps, this line on the inside of the 

 head furnishes a foundation for a septum, which in the sides of the 

 nose is very high, and connects also with the hind part of the head. 



